There exists a class of machinery which utilizes mechanical resonance as the means to obtain periodic motion of the machine's elements. For convenience, the resonant machine of this invention will be referred to as a Resonant Piston Compressor (RPC). The RPC is a reciprocating compressor which falls into this class of machinery and which can be utilized in various compressor applications, such as for example electrically-driven heat pumps. Generally, an RPC is comprised of an electrodynamic motor which drives a reciprocating piston and thereby provides the compression action on a working fluid which may be a gas or a liquid.
In known free piston resonant reciprocating compressors the fluid compressing member, such as a piston, is driven by a suitable motor, such as a linear reciprocating electrodynamic motor. A compression piston is usually coupled to the motor armature and the armature is held in a rest position by way of one or more main or resonance springs. When the motor is energized, such as by an alternating current, a magnetic force is generated to drive the piston and the resonance spring causes the piston to oscillate back and forth to provide compression of the fluid.
It is desirable to provide an RPC which operates over a broad range of suction and discharge pressures such as that which occurs in a residential heat pump.
The advantage of the RPC in such an application is that variable capacity heat pump operation can be achieved by modulating the RPC's piston stroke. In conventional heat pumps, variable capacity operation is generally achieved by modulating speed of the compressor by varying the electrical frequency supplied to the compressor motor. However, the cost of the solid-state electronic components required to achieve a variable frequency motor drive would be appreciably higher than the cost of the components needed for a fixed-frequency variable current drive for a modulating RPC.
Unfortunately, the heat pump application does not have a fixed design-point condition. The compressor inlet and discharge pressures change with changes in outdoor temperature. For example, at an outdoor temperature of 95.degree. F., the compressor inlet and discharge pressures for R-22 refrigerant will typically be 90 and 300 psia, while on a 15.degree. F. day the inlet and discharge pressures will typically drop to 37 and 200 psia, respectively. As a result of this variation in pressures, there is a significant variation in the "gas-spring stiffness" of both the compressor and the balance chamber with outdoor temperature. This in turn causes a significant variation in resonant tuning of the RPC to the point where satisfactory operation of the RPC is not possible at one or the other outdoor temperature extreme.
For example, in the case of a 21/2 ton rated compressor, the following table shows the variation in compressor and balance cylinder stiffness for outdoor temperatures of 95.degree. F. and 12.degree. F.
______________________________________ Stiffness (lbf/in) Outdoor Compressor Balance Total Temperature (.degree.F.) Cylinder Cylinder Stiffness ______________________________________ 95 1735 228 1963 12 1019 126 1145 ______________________________________
There is roughly a 40 percent reduction in total stiffness at the 12.degree. F. outdoor temperature condition compared to the 95.degree. F. condition. This is too great a reduction for a fixed frequency, resonantly operating RPC.
More particularly, for a plunger of 6 pounds, and a natural frequency of 60 Hertz, the required gas-spring stiffness of the unit is approximately 2000 lbf/in. During air-conditioning operation on a 95.degree. F. day, approximately 90 percent of this required stiffness is supplied by the compression chamber. The remaining 10 percent must be supplied by the balance chamber. However, during heating operation on a 12.degree. F. day, the compression chamber will provide only 60 percent of the required stiffness due to the reduced pressure level of the R-22 refrigerant cycle. The remaining 40 percent must be supplied by the balance chamber. However, for the same reason that stiffness of the compression chamber is reduced on a 12.degree. F. day, also so will the stiffness of the balance chamber be reduced unless some other means is available to counter this reduction.
In co-pending patent application Ser. No. 014,444, filed Feb. 13, 1987, and assigned to the same assignee as the present invention, there is described and claimed an improved resonant piston reciprocating compressor which operates over a broad range of suction and discharge pressures such as occurs in a residential heat pump application, for example.
In the arrangement described and claimed in the foregoing patent application the effect of reduced gas spring stiffness during the heating mode of operation is countered by providing means associated with the balance chamber for changing the total balance chamber volume between the heating and cooling modes of operation. In one arrangement of the foregoing patent application a double-sided piston means reciprocates within a cylinder and defines a valved compression space on one side of the piston means and a balance chamber on the other side thereof. The arrangement includes a movable head means adjustable between first and second positions to effect a large change in the total volume of the balance chamber between the two positions. That is, in one position of the movable head, an extended balance chamber volume is coupled to the balance chamber while in the other position of the movable head the extended balance chamber volume is decoupled from the balance chamber.
As further background the disclosure of the foregoing referenced patent application is incorporated herein by reference.